Elephant sharks' DNA hasn't changed much in the 420 million years that bony fishes have been in existence, suggests a new analysis of the animal's genome.

Callorhinchus milii is the first cartilaginous fish to have its genome fully sequenced. At 1 billion base pairs, the elephant shark has the smallest genetic code among sharks, rays and other similar fishes. A closer look at the animal's genome suggests that the fish do not have many bones because they lack genes that switch on calcium-binding proteins, researchers report January 8 in Nature.

Elephant sharks also lack a major component of the immune system called helper T cells. The discovery suggests that acquired immunity evolved in two steps — not one, as previously thought.